Whenever you summon or create a charged creeper, then spawn in a regular creeper near it, part of the regular creeper's texture will be transparent depending on the angle that you look at it. I'm not too sure if this effects every creeper in the world or just the loaded chunks, but I have seen this effect on blazes as well. I couldn't get the screenshot though. It only worked once.

Steps to reproduce:
1. Spawn a charged creeper or strike a creeper with lightning.
2. spawn a regular creeper next to it. (worked at about 5 blocks away)
3. move around a bit until you see the regular creeper's transparent texture.